a) Background. The work in my lab has revolved around the study of chondrocyte gene expression and viability with attention focused on the changes that take place in human cartilage that lead to the arthritic diseases. We have sought to identify and analyze the factors that affect these aspects of cartilage biology. We have explored the roles of histone deacetylases in regulating chondrocyte viability and gene expression. We have found that distinct histone deacetylase are potent regulators of both viability and cartilage specific gene expression. These results provide us with the background to our present studies, enabling us to fill the gaps in our knowledge about the role of critical intracellular proteins in cartilage biology. b) Objectives of Present Studies. Currently our objectives are two-fold. The first objective is to define the role of histone deacetylaseses in chondrocyte gene epxression. The second objective is to identify the mechanism by which specific histone deacetylase enzymes regulate chondrocyte survival. Results During the Past Year. The results of the past year are grouped according to the Objectives outlined above. (1) Analysis of the role of chromatin modifying enzymes in cartilage specific gene expression. We have been able to identify the most abundant histone deacetylase enzymes (HDACs) that are expressed in primary human chondrocytes. We find that these enzymes are potent regulators of chondrocyte specific gene expression such that when select HDACs are overexpressed in human chondrocytes they can act as potent enhancers or inhibitors of cartilage gene expression. For example, SirT1 is a class III histone deacetylase that can extend organism life span during caloric restriction. However, little is know of the role it plays in human chondrocyte biology. We find that the SirT1 plays a very positive role in the regulation of both cartilage expression and chondrocyte survival. Further, we find that SirT1 protein levels are downregulated in the chondrocytes derived from osteoarthritic patients, compared to normal donors. Thus SirT1 is an important new marker for OA. In addition, we find that other HDACs, namely HDAC1 and HDAC2, act in an opposite way to SirT1 in that they are potent repressors of cartilage gene expression. Further we find that HDAC1 and HDAC2 are elevated in OA cartilage. Thus a conclusion from our studies is that a misregulation of HDAC enzymes may contribute to the onset of OA. (2) Analysis of the role of Sirt1 in chondrocyte survival. We find that SirT1 is a potent regulator of chondrocyte survival. When SirT1 is overexpressed in human chondrocytes it leads to activation of the IGF/Akt pathway, leading to a block to apoptosis. SirT1 accomplishes this function in part by repressing protein tyrosine phosphatase 1B (PTP1B) (d) Conclusions and Significance With Future Directions. In the last two to three decades, much progress has been made in the discovery of the causes and treatments of the various forms of arthritis. However, the initiating events for these diseases are not well understood and many of the treatment regimens are suboptimal at best. Since arthritis is a very debilitating disease that affects a large proportion of the population, much effort should be made at uncovering the root cause of the disease and identifying new treatments. I believe we have made excellent progress in defining new areas of research in cartilage biology and potential treatments of arthritic diseases. We have explored the role of HDACs and their corresponding inhibitors in chondrocyte gene expression and as a potential treatment of OA. The role of HDACs in cartilage biology is a completely untouched area that holds much promise for a better understanding of the regulatory mechanisms underlying chondrocyte gene expression and in the treatment of cartilage diseases through the use of specific HDAC inhibitors.